Semiconductor light emitting device with II-VI group semiconductor contact layer containing alkali metal impurity, method of producing the same, and optical device including same

ABSTRACT

The invention provides a semiconductor light emitting device whose operating voltage can be easily reduced, a method of producing the same, and an optical device. An n-type clad layer, a first guide layer, an active layer, a second guide layer, a p-type clad layer, a first semiconductor layer, and a second semiconductor layer of ZnSe are successively grown on an n-type substrate. An alkali compound layer of Na 2 Se is then formed thereon. Subsequently, a heat treatment is performed by means of irradiation of an excimer laser beam so that at least a part of the second semiconductor layer and at least a part of the alkali compound layer are altered thereby forming a contact layer. Furthermore, a p-side electrode is formed on the contact layer. The contact layer contains an alkali metal serving as a p-type impurity so that the contact layer has a low electric resistance thereby achieving a reduction in the operating voltage and thus a reduction in the operating power. As a result of the reduction in the operating power, the device life is improved.

RELATED APPLICATION DATA

This application is a divisional of copending application Ser. No.09/048,048 filed Mar. 26, 1998. The present and foregoing applicationclaims priority to Japanese application Nos. P09-076157 filed Mar. 27,1997 and No. P09-247326 filed Sep. 11, 1997. All of the foregoingapplications are incorporated herein by reference to the extentpermitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light emitting devicehaving a multilayer structure comprising at least an n-type clad layer,an active layer, and a p-type clad layer each made of a II-VI groupcompound semiconductor, a method of producing the same, and an opticaldevice provided with the same.

2. Description of the Related Art

In recent years, there is an increasing demand for high density and highresolution in recording and reproducing of data onto and from an opticaldisk or a magneto-optic disk. There are also attempts to develop ahigh-intensity display device, a low-loss optical fiber communicationsystem, and an optical analysis instrument for analyzing DNA or specialchemicals. Thus there is a need to develop a semiconductor lightemitting device for use as a light source in these applications which iscapable of emitting light with a color in the range of green and blue.

Promising candidates as materials used to form a semiconductor devicecapable of emitting light with a color in the range of green to blue areII-VI compound semiconductors containing at least one II-group elementselected from the group consisting of zinc (Zn), magnesium (Mg),beryllium (Be), cadmium (Cd), mercury (Hg), and manganese (Mn) and atleast one VI-group element selected from the group consisting of oxygen(O), sulfur (S), selenium (Se), and tellurium (Te). However, such aII-VI semiconductor light emitting device has a high Schottky barrier atinterfaces between electrodes and semiconductors (refer to for exampleI. Suemune, Appl. Phys. Lett. 63 (1993) 2612), and thus has a highcontact resistance which results in a high operating voltage. ZnSe iswidely used as a layer in contact with an electrode. However, it isdifficult to dope a p-type impurity into ZnSe to a high enough level toobtain a high enough carrier concentration. This makes it difficult torealize a good ohmic contact, and the result is a high operatingvoltage. As a result, high power dissipation occurs, which results inheat generation which in turn causes degradation of the device.

One technique proposed to avoid the above problem is to form anadditional ZnTe layer on the ZnSe layer in such a manner that the ZnTelayer is doped with a p-type impurity to a higher level than that of theZnSe layer so that the highly-doped ZnTe layer serves as a contact layerwith the p-side electrode. However, if the ZnSe layer is grown directlyon the ZnTe layer, a great valence-band discontinuity occurs at theinterface between the ZnSe and ZnTe layers. This great valence-banddiscontinuity results in a high resistance which makes it impossible toachieve a low operating voltage. One technique proposed to solve theabove problem is to employ a graded ZnSeTe layer in which the Se-Tecomposition ratio is gradually changed, thereby reducing the operatingvoltage (refer to for example Y. Fan, D. C. Grillo, M. D. Rinqle, J.Han, L. He, R. L. Gunshor, A. Salokatve, H. Jeon, M. Hovinen, A. V.Nurmikko, G. C. Hua and N. Otsuka, J. Vac. Sci. Technol. B12 (1994)2480). Another technique proposed for the same purpose is to dispose asuperlattice layer consisting of ZnSe and ZnTe between the ZnSe and ZnTelayers (refer to for example F. Hiei, M. Ikeda, M. Ozawa, T. Miyajima,A. Ishibashi and K. Akimoto, Electron. Lett., 29 (1993) 878; JapanesePatent Laid-Open No. 6-5920).

The above techniques in which a composition-graded layer or asuperlattice layer is employed have the problem that it is difficult toform such a layer with high crystal quality. ZnSe has a lattice constantof 5.66942 Å while the lattice constant of ZnTe is 6.10 Å. Such a largedifference in the lattice constant causes introduction of misfitdislocations when a ZnTe layer is grown on a ZnSe layer. Misfitdislocations act as hole traps which cause a reduction in theconcentration of p-type carriers which in turn results in an increase inthe operating voltage. As a result, it is impossible to achieve asufficient reduction in the power dissipation and thus it is impossibleto avoid the degradation of the device to a sufficient degree.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a semiconductor light emitting device whose operating voltagecan be easily reduced to a low enough level, a method of producing sucha light emitting device, and an optical device provided with such alight emitting device.

According to an aspect of the invention, there is provided asemiconductor light emitting device comprising at least an n-type cladlayer, an active layer, and a p-type clad layer formed into a multilayerstructure using a II-VI compound semiconductor containing at least oneII-group element selected from the group consisting of zinc, magnesium,beryllium, cadmium, manganese, and mercury and at least one VI-groupelement selected from the group consisting of oxygen, sulfur, selenium,and tellurium, the semiconductor light emitting device also comprising ap-side electrode electrically connected to the p-type clad layer, thesemiconductor light emitting device being characterized in that acontact layer is provided between the p-type clad layer and the p-sideelectrode, the contact layer being formed, at least in part, of a II-VIgroup semiconductor containing an alkali metal element serving as ap-type impurity.

According to another aspect of the invention, there is provided asemiconductor light emitting device comprising at least an n-type cladlayer, an active layer, and a p-type clad layer formed into a multilayerstructure using a II-VI compound semiconductor containing at least oneII-group element selected from the group consisting of zinc, magnesium,beryllium, cadmium, manganese, and mercury and at least one VI-groupelement selected from the group consisting of oxygen, sulfur, selenium,and tellurium, the semiconductor light emitting device also comprising ap-side electrode electrically connected to the p-type clad layer,wherein there is provided a contact layer between the p-type clad layerand the p-side electrode, the contact layer containing a product ofthermal reaction between an alkali compound and a II-VI group compoundsemiconductor, or containing an alkali compound and a product of thermalreaction between an alkali compound and a II-VI group compoundsemiconductor.

According to still another aspect of the invention, there is provided amethod of producing a semiconductor light emitting device comprising atleast an n-type clad layer, an active layer, and a p-type clad layerformed into a multilayer structure using a II-VI compound semiconductorcontaining at least one II-group element selected from the groupconsisting of zinc, magnesium, beryllium, cadmium, manganese, andmercury and at least one VI-group element selected from the groupconsisting of oxygen, sulfur, selenium, and tellurium, the methodcomprising the steps of: forming a plurality of II-VI group compoundsemiconductor layers into a multilayer structure, the semiconductorlayers including at least the n-type clad layer, the active layer, andthe p-type clad layer; forming an alkali compound layer on the surfaceof a II-VI group compound layer located on the side, adjacent to thep-type clad layer, of the active layer; after said step of forming thealkali compound layer, forming a p-side electrode in an areacorresponding to the alkali compound layer;

According to still another aspect of the invention, there is provided anoptical device including a semiconductor light emitting device, thesemiconductor light emitting device comprising at least an n-type cladlayer, an active layer, and a p-type clad layer formed into a multilayerstructure using a II-VI compound semiconductor containing at least oneII-group element selected from the group consisting of zinc, magnesium,beryllium, cadmium, manganese, and mercury and at least one VI-groupelement selected from the group consisting of oxygen, sulfur, selenium,and tellurium, the semiconductor light emitting device also comprising ap-side electrode electrically connected to the p-type clad layer,wherein a contact layer is provided between the p-type clad layer andthe p-side electrode, the contact layer being formed, at least in part,of a II-VI group semiconductor containing an alkali metal elementserving as a p-type impurity.

According to still another aspect of the invention, there is provided anoptical device including a semiconductor light emitting device, thesemiconductor light emitting device comprising at least an n-type cladlayer, an active layer, and a p-type clad layer formed into a multilayerstructure using a II-VI compound semiconductor containing at least oneII-group element selected from the group consisting of zinc, magnesium,beryllium, cadmium, manganese, and mercury and at least one VI-groupelement selected from the group consisting of oxygen, sulfur, selenium,and tellurium, the semiconductor light emitting device also comprising ap-side electrode electrically connected to the p-type clad layer,wherein there is provided a contact layer between the p-type clad layerand the p-side electrode, the contact layer containing a product ofthermal reaction between an alkali compound and a II-VI group compoundsemiconductor, or containing an alkali compound and a product of thermalreaction between an alkali compound and a II-VI group compoundsemiconductor.

In the semiconductor light emitting device according to the presentinvention, when a voltage is applied between the n-side electrode andthe p-side electrode, a current is injected into the active layer viathe contact layer. As a result of the injection of the current, emissionof light occurs in the active layer. Because the contact layer is dopedwith an alkali metal element so that it has a low electric resistance,the electric resistance at the interface between the p-side electrodeand the contact layer becomes low and thus the voltage drop across theinterface also becomes low. As a result, the semiconductor lightemitting device can be operated with low electric power withoutgenerating a significantly great amount of heat, and thus a long devicelife can be achieved.

In the semiconductor light emitting device according to another aspectof the present invention, when a voltage is applied between the n-sideelectrode and the p-side electrode, a current is injected into theactive layer via the contact layer. As a result of the injection of thecurrent, emission of light occurs in the active layer. Because thecontact layer is formed to have a low electric resistance by means ofincorporating a product of thermal reaction between an alkali compoundand a II-VI group compound semiconductor, the electric resistance at theinterface between the p-side electrode and the contact layer becomes lowand thus the semiconductor light emitting device can be operated withlow electric power.

In the semiconductor light emitting device according to still anotheraspect of the present invention, a plurality of II-VI group compoundlayers including at least an n-type clad layer, an active layer, and ap-type clad layer are formed into a multilayer structure, and then analkali compound layer is formed on the surface of a II-VI group compoundlayer located on the side, adjacent to the p-type clad layer, of theactive layer. After that, a p-side electrode is formed in an areacorresponding to the alkali compound layer.

Furthermore, since the optical device according to the invention has asemiconductor light emitting device according to the invention, thesemiconductor light emitting device can be operated such that light isemitted in its active layer by applying a small voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the structure of asemiconductor light emitting device according to a first embodiment ofthe invention;

FIGS. 2A and 2B are cross-sectional views illustrating processing stepsof producing the semiconductor light emitting device shown in FIG. 1;

FIGS. 3A and 3B are cross-sectional views illustrating the processingsteps following those shown in FIGS. 2A and 2B;

FIGS. 4A and 4B are cross-sectional views illustrating the processingsteps following those shown in FIGS. 3A and 3B;

FIG. 5 is a schematic diagram illustrating an MBE crystal growingapparatus used to produce the semiconductor light emitting device shownin FIG. 1;

FIG. 6 is a schematic diagram illustrating an energy beam irradiatingapparatus used to produce the semiconductor light emitting device shownin FIG. 1;

FIG. 7 is a schematic diagram illustrating a vacuum apparatus used toproduce the semiconductor light emitting device shown in FIG. 1;

FIGS. 8A and 8B are cross-sectional views illustrating processing stepsof producing the semiconductor light emitting device shown in FIG. 1,according to another production method;

FIGS. 9A and 9B are cross-sectional views illustrating the processingsteps following those shown in FIGS. 8A and 8B;

FIG. 10 is a graph illustrating the dependence of the electricresistance on the number of laser beam irradiation pulses;

FIG. 11 is a graph illustrating the normal current range ofcurrent-voltage characteristic of the semiconductor light emittingdevice subjected to the heating and drying process according to theproduction method of the present invention;

FIG. 12 is a graph illustrating the normal current range ofcurrent-voltage characteristic of the semiconductor light emittingdevice subjected to no heating and drying process, which was preparedfor the purpose of comparison;

FIG. 13 is a graph illustrating the current-voltage characteristicbetween p-side electrodes of adjacent semiconductor light emittingdevices subjected to the heating and drying process according to theproduction method of the present invention;

FIG. 14 is a graph illustrating the current-voltage characteristicbetween p-side electrodes of adjacent semiconductor light emittingdevices subjected to no heating and drying process, which were preparedfor the purpose of comparison;

FIG. 15 is a graph illustrating the current-voltage characteristic of asemiconductor light emitting device subjected to two-stage-energy beamirradiation in the heat treatment process according to the productionmethod of the invention;

FIG. 16 is a graph illustrating the current-voltage characteristicbetween p-side electrodes of adjacent semiconductor light emittingdevices subjected to two-stage energy beam irradiation in the heattreatment process according to the production method of the invention;

FIG. 17 is a cross-sectional view illustrating the structure of asemiconductor light emitting device according to a second embodiment ofthe invention;

FIG. 18 is a cross-sectional view illustrating the structure of asemiconductor light emitting device according to a third embodiment ofthe invention;

FIG. 19 is a cross-sectional view illustrating the structure of asemiconductor light emitting device according to a fourth embodiment ofthe invention;

FIG. 20 is a cross-sectional view illustrating the structure of asemiconductor light emitting device according to a fifth embodiment ofthe invention; and

FIG. 21 is a schematic diagram illustrating an optical disk reproducingapparatus according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in further detail below withreference to preferred embodiments in conjunction with the accompanyingdrawings.

FIG. 1 illustrates the structure of a first embodiment of asemiconductor light emitting device according to the present invention.The semiconductor light emitting device comprises a substrate 1 andsemiconductor layers successively formed thereon one on another in thefollowing order: a III-V buffer layer 2, a first II-VI group bufferlayer 3 of a II-VI compound semiconductor, a second II-VI group bufferlayer 4 of a II-VI compound semiconductor, an n-type clad layer 5, afirst guide layer 6, an active layer 7, a second guide layer 8, a p-typeclad layer 9, a first semiconductor layer 10, and a second semiconductorlayer 11.

The substrate 1 is for example made of n-type GaAs having a thickness of100 to 350 μm and doped with silicon (Si) acting as an n-type impurity.The III-V group buffer layer 2 is for example made of n-type GaAs havinga thickness of 200 nm measured in the direction of growth (hereinafterreferred to simply as the thickness) and doped with silicon serving asan n-type impurity.

The first II-VI group buffer layer 3 is for example made of n type ZnSehaving thickness of 20 nm and doped with chlorine (Cl) serving as ann-type impurity. The second II-VI group buffer layer 4 is for examplemade of n type ZnSSe having a thickness of 200 nm and doped withchlorine serving as an n-type impurity. The n-type clad layer 5 is forexample made of an n-type ZnMgSSe mixed crystal having a thickness of 1μm and doped with chlorine serving as an n-type impurity.

The first guide layer 6 is for example made of a ZnSSe mixed crystalhaving a thickness of 100 nm and doped with chlorine serving as ann-type impurity or doped with no impurity. In the ZnSSe mixed crystal,the composition ratios of VI-group elements are selected as 6% for S and94% for Se so that the ZnSSe mixed crystal has a lattice constantmatched with that of GaAs used to form the substrate 1.

The active layer 7 is formed, for example, into a single quantum wellstructure with a thickness of 6 nm using a ZnCdSe mixed crystal. In theZnCdSe mixed crystal, the composition ratios of II-group elements areselected as 75% for Zn and 25% for Cd so that the ZnCdSe mixed crystalhas a lattice constant slightly greater than that of GaAs used to formthe substrate 1.

The second guide layer 8 is for example made of a ZnSSe mixed crystalhaving a thickness of 100 nm and doped with nitrogen (N) serving as ap-type impurity or doped with no impurity. In the ZnSSe mixed crystal,the composition ratios of VI-group elements are selected as 6% for 5 and94% for Se.

The p-type clad layer 9 is for example made of a p-type ZnMgSSe mixedcrystal having a thickness of 1 μm and doped with nitrogen serving as ap-type impurity. The first semiconductor layer 10 is for example made ofa p-type ZnMgSSe mixed crystal having a thickness of 1 μm and doped withnitrogen serving as a p-type impurity. The second semiconductor layer 11is for example made of p-type ZnSe having a thickness of 100 nm anddoped with nitrogen serving as a p-type impurity.

On the second semiconductor layer 11, there is provided a contact layer12 having a thickness for example of 10 to 1000 nm. The contact layer 12is made of a II-VI group compound semiconductor such as ZnSe containing,at least in a part thereof, a product of thermal reaction of the secondsemiconductor (ZnSe in this case) with an alkali compound, or containingan alkali compound as well as such a thermal reaction product. That is,the contact layer 12 is formed of a II-VI group compound semiconductor(ZnSe in this case) containing, at least in a part thereof, an alkalimetal element serving as a p-type impurity. The alkali metal is dopedfor example at 1×10¹⁸ to 1×10²¹ atoms/cm³ so that the contact layer 12has a carrier concentration as high as about 1×10¹⁹ 1/cm³.

Alkali compounds preferable for this purpose include compounds of analkali metal and a VI-group element, an alkali metal and phosphorus (P),and an alkali metal, VI-group element, and phosphorus. More preferably,the compound contains at least one alkali metal selected from the groupconsisting of sodium (Na) and potassium and at least one VI-groupelement selected from the group consisting of oxygen, sulfur, selenium,tellurium, and phosphorus. More specifically, the preferable compoundsfor this purpose include Na₂S, Na₂Se, Na₂O, Na₂O₂, Na₂Te, K₂S, K₂Se,NaPO₃, Na₃P, and K₃P.

The second semiconductor layer 11 and the contact layer 12 are formedinto a stripe shape with a width of for example 10 μm which serves as acurrent confining part in which a current is confined. On the firstsemiconductor layer 10, in areas where the second semiconductor layer 12and the contact layer 12 are not formed, there is provided an insulatinglayer 13 made of for example aluminum oxide (Al₂O₃).

On the insulating layer 13 and the contact layer 12, there is provided ap-side electrode 14 formed of a proper metal (such as gold (Au) or ametal containing gold) with a thickness of for example about 200 nm,wherein the p-side electrode 14 is electrically connected to the p-typeclad layer 9 via the contact layer 12, the second semiconductor layer11, and the first semiconductor layer 10. Although in the specificexample shown in FIG. 1, the p-side electrode 14 is formed over theentire surface of the insulating layer 13 and the contact layer 12, thep-side electrode 14 may by formed only in a particular areacorresponding to the contact layer 12 (so that all the area above thecontact layer 12 and a partial area of the insulating layer 13 in thevicinity of the contact layer 12 are covered with the p-side electrode).On the back surface of the substrate 1, there is provided an n-sideelectrode 15 formed of for example indium (In), which is electricallyconnected to the n-type clad layer 5 via the substrate 1, the III-Vgroup buffer layer 2, the first II-VI group buffer layer 3 and thesecond II-VI group buffer layer 4.

Although not shown in the figure, the semiconductor light emittingdevice also has two reflecting mirror layers formed on the respectivetwo opposite sides perpendicular to the longitudinal direction of thecontact layer 12 (i.e., perpendicular to the longitudinal direction ofthe resonator). The reflecting mirror layers are formed in a multilayerstructure in which for example aluminum oxide films and silicon film arealternately disposed.

The semiconductor light emitting device having the structure describedabove may be produced as follows.

The production process for this purpose is shown in FIGS. 2 to 4. First,as shown in FIG. 2A, a plurality of semiconductor layers are formedsuccessively on the substrate 1 using a molecular beam epitaxy (MBE)technique so as to form a multilayer structure consisting of the III-Vgroup buffer layer 2, the first II-VI group buffer layer 3, the secondII-VI group buffer layer 4 of a II-VI compound semiconductor, the n-typeclad layer 5, the first guide layer 6, the active layer 7, the secondguide layer 8, the p-type clad layer 9, the first semiconductor layer10, and the second semiconductor layer 11 (multilayer formation step).

FIG. 5 illustrates the structure of the MBE crystal growing apparatusused herein. The MBE crystal growing apparatus has a vacuum chamber 21in which there is provided a substrate holder 22 for holding a substrate1. The vacuum chamber 21 has a plurality of particle beam generationcells (such as Knudsen cells) disposed so that they are directed to thesubstrate 1. The vacuum chamber 21 also has a plasma generation chamber24 for generating a nitrogen plasma toward the substrate 1. The plasmageneration chamber 24 comprises, for example, an ECR (electron cyclotronresonance) cell or an RF (radio frequency) cell. Shutters are disposednear the emission apertures of the respective particle beam generationcells 23 and also near the aperture of the plasma generation chamber 24whereby the irradiation of each particle beam is controlled.

Using the MBE crystal growing apparatus having the above structure,particle beams of various source materials depending on the compositionsof the compound semiconductors of the respective layers are emitted fromthe respective particle beam generation cells 23 so that the substrateis exposed to these particle beams thereby epitaxially growing therespective layers. The doping of silicon into the III-V group bufferlayer 2 is performed by exposing the layer to a silicon particle beamtogether with the particle beams of the required source materialsemitted from the corresponding particle beam generation cells 23. Thedoping of chlorine into the first II-VI group buffer layer 3, the secondII-VI group buffer layer 4, the n-type clad layer 5, and the first guidelayer 6, respectively, is performed by exposing the layers to a chlorineparticle beam together with the particle beams of the required sourcematerials emitted from the corresponding particle beam generation cells23. On the other hand, the doping of nitrogen into the second guidelayer 8, the p-type clad layer 9, the first semiconductor layer 10, andthe second semiconductor layer 11 is performed by exposing the layers toa nitrogen plasma generated by the plasma generation chamber 24 as wellas to the respective particle beams of source materials. Preferably, theIII-V group buffer layer 2 and the II-VI group compound semiconductorlayer thereon are formed separately in different vacuum chambersconnected to each other via an ultra-high vacuum transfer path so as toprevent stacking faults being created at the interface between the III-Vgroup buffer layer 2 and the II-VI group compound semiconductor layer.

Then as shown in FIG. 2B, a resist film M is coated on the secondsemiconductor layer 11, and a mask pattern consisting of a plurality ofparallel stripes is formed in the resist film M by means ofphotolithography. The second semiconductor layer 11 is then selectivelyremoved by means of wet or dry etching using the resist film M as a maskthereby converting the second semiconductor layer 11 into the form of aplurality of parallel stripes so that they act as current confiningparts. In FIG. 2B, only one of these stripes of the second semiconductorlayer 11 is shown in a representative fashion (FIGS. 3 and 4 are alsorepresented in a similar fashion). Subsequently, as shown in FIG. 3A, aninsulating material such as aluminum oxide is evaporated over the entiresurface (on the resist film M and the first semiconductor layer 10 whichhas appeared as a result of the selective removal of the secondsemiconductor layer 11). The resist film M is then removed (lifted off)together with the part of the insulating material evaporated on theresist film M thereby forming an insulating layer 13 (current confiningpart formation step).

After that, the surface of the second semiconductor layer 11 and that ofthe insulating layer 13 are cleaned (cleaning step). This cleaningprocess is needed to remove impurities (such as organic substances orcarbon compounds) from the surface. If there is a residual impurity, acompound including the impurity is formed in the contact layer 12 in alater process (in which an alkali compound layer is formed on the secondsemiconductor layer 11 and a heat treatment is performed). The formationof such a compound of impurity would cause nonuniformity of current andthus degradation of current-voltage characteristic.

The cleaning process preferably includes the steps of cleaning organicimpurities with acetone or the like and then performing cleaning withflowing water to an enough extent. After the cleaning with flowingwater, chemical etching may be performed using an acid or alkalineetchant capable of dissolving the surface of the second semiconductorlayer 11 so that a 10 Å surface portion is removed from the secondsemiconductor layer 11. In the device structure in which currentconfining parts are formed as is the case in the present embodiment, thecleaning process is particularly important because there is apossibility that the surface of the stripe-shaped second semiconductorlayer 11 is contaminated.

After cleaning the surface, drying is performed at 120° C. for 10 min ina dry ambient (for example nitrogen (N₂) ambient) so that water andimpurities remaining on the surface are removed (heating and dryingstep). If there is residual water, when an alkali compound is evaporatedon the surface of the second semiconductor layer 11 in a later step(alkali compound layer formation step), water is absorbed by the alkalicompound having the property of deliquescence. The absorption of waterbrings about a bad influence on the formation of the interface betweenthe contact layer 12 and the p-side electrode 14, and degradation occursin the current-voltage characteristic. If there is also a residualimpurity on the surface, the impurity is incorporated together withwater and a compound including the impurity is formed partially in thecontact layer 12.

After the heating and drying process, as shown in FIG. 3B, an alkalicompound (such as Na₂Se) is deposited on the second semiconductor layer11 and the insulating layer 13 by means of vacuum evaporation in avacuum evaporation apparatus not shown in the figure thereby forming thealkali compound layer 16 a with a thickness of about 20 nm (alkalicompound layer formation step). The thickness of the alkali compoundlayer 16 a may be smaller or greater than the above value. In thethicker case, an excessive part of the alkali compound will disappear ina heating process performed later, and thus no problem occurs. Thealkali compound layer 16 a may be formed either over the entire surfaceof the second semiconductor layer 11 and the insulating layer 13 asshown in FIG. 3B, or in a particular limited area corresponding to thesecond semiconductor layer 11.

Because the alkali compound is deliquescent, it is preferable to performthe alkali compound layer formation process in a dry ambient. In thisprocess, before performing evaporation of the alkali compound in thevacuum evaporation apparatus (not shown in the figure), the contaminantremaining on the surface may be removed by heating the substrate to atemperature in the range of 80° C. to 300° C. using a heater or by meansof irradiation of an excimer laser beam or an electron beam.

After the formation of the alkali compound layer 16 a, a heat treatmentis performed in a proper manner (for example by means of irradiation ofan energy beam) thereby altering a part, on the side adjacent to thealkali compound layer 16 a, of the second semiconductor layer 11 and atleast a part, in contact with the second semiconductor layer 11, of thealkali compound layer 16 a thus forming the contact layer 12 as shown inFIG. 4A (heat treatment step). By way of example, the heat treatment isdescribed in further detail below for the case where the heat treatmentis performed by means of irradiation of an energy beam.

FIG. 6 illustrates the structure of an energy beam irradiating apparatusused herein. The energy beam irradiating apparatus has a reactionchamber 31 in which there is provided a susceptor on which a substrate 1is placed. The susceptor 32 can be heated by a heater which is not shownin the figure. The reaction chamber 31 has a gas exhaust pipe 34 and agas supply pipe 33 through which nitrogen gas is supplied into thereaction chamber 31 from a nitrogen gas cylinder 35 via a nitrogen gaspurifier 36. The gas supply pipe 33 and the gas exhaust pipe 34 havetheir own valve 37. The reaction chamber 31 is also provided with apressure gauge 38 for measuring the pressure in the reaction chamber 31.

The energy beam irradiating apparatus also includes an energy beamgenerator 41 for generating an energy beam toward a substrate 1 placedin the reaction chamber 31. The energy beam generator 41 comprises forexample an excimer laser. The energy beam generated by the energy beamgenerator 41 is collimated by a lens 42 and then reflected by areflecting plate 43 toward the substrate 1 so that the substrate 1 isirradiated with the energy beam.

The substrate 1 on which the alkali compound layer has been formed istaken out of the vacuum evaporation apparatus which is not shown in thefigure, and it is placed into the energy beam irradiating apparatus.After quickly evacuating the energy beam irradiating apparatus, theapparatus is filled with an inert gas (such as argon (Ar), helium (He)or nitrogen gas). Furthermore, the evacuation and the supply of theinert gas or nitrogen gas are performed repeatedly so that the apparatusis finally filled with the inert gas or nitrogen at a pressure higherthan normal atmospheric pressure (preferably at a pressure in the rangeof 2 to 10 atm). The high pressure in the apparatus is needed to preventthe alkali compound layer from being vaporized and lost during theirradiation of the energy beam. The reason why an inert gas or nitrogengas is employed as a gas with which the inside of the apparatus isfilled is that the inert gas is stable and that nitrogen gas behaves asa p-type impurity in II-VI group compound semiconductors. Although notshown in the figure, a cover made of a material such as quartztransparent to the energy beam may be placed on the substrate wherebythe alkali compound layer is prevented from being evaporated and lost.

After filling the inside of the apparatus with the high-pressure ambientgas, the surface of the alkali compound layer (not shown) is irradiatedwith for example a pulse of an excimer laser beam. The wavelength of thelaser beam may be properly selected from various values. Morespecifically, 248 nm or other wavelengths of KrF, 193 nm of ArF, or 308nm of XeCl may be employed. Although there is no limitation in thewavelength, it is preferable that the wavelength be as short as theabove examples. When the wavelength is short enough, it is possible toheat the substrate for only the limited part near the surface of thealkali compound layer. In contrast, when a long wavelength is employed,a wider region including a portion near the active layer 7 is heated.The result in this case is bad crystal quality and degradation in thecharacteristics of the device.

The pulse width and the output power (i.e., the energy) of the beam maybe properly determined. In this specific embodiment, the pulse width isset to 20 nsec, and the output power is set, in the case where 248 nm ofKrF is employed, to 20-90 mJ/cm² measured at the surface of the alkalicompound layer. If the output power is too low, the alkali metal atomsin the alkali compound cannot be diffused into the second semiconductorlayer 11. Conversely, if the output power is too high, the surface ofthe second semiconductor layer 11 is damaged and degradation occurs inthe current-voltage characteristic. If irradiation of a beam having ahigh power is performed repeatedly many times, a reduction occurs in thenumber of carriers in the p-type II-VI semiconductor layers such as thefirst semiconductor layer 10 and the second semiconductor layer 11 dopedwith nitrogen serving as a p-type impurity, and thus the resistance ofthese layers becomes high. If the output power is still higher, there isa possibility that the current confining part, if there is such a partas is the case in this embodiment, is separated from the lower portion.The output power described herein is a value optimum for the wavelengthof 248 nm, and the optimum output power varies with the wavelengthemployed.

The irradiation of the beam may be performed once or more times at thesame output power level, or may be performed a plurality of times atdifferent output power levels. In the case where the irradiation isperformed at different output power levels, it is preferable thatirradiation be performed at least once (more preferably a plurality oftimes) using a low-power beam at the first stage, and then furtherperformed at least once using a high-power beam at the second stage. Atthe first stage, the irradiation of the low-power beam causes alkalimetal atoms in the alkali compound layer to penetrate into the secondsemiconductor layer 11. At the second stage, the irradiation of thehigh-power beam causes the alkali metal atoms to diffuse into the secondsemiconductor layer 11. The irradiation of the high-power beam at thesecond stage also causes the alkali compound layer remaining on thesurface to be evaporated and thus removed.

In the above beam irradiation process, the substrate 1 may be maintainedat room temperature or may be heated via the susceptor 32 to a propertemperature, and the beam power may be adjusted depending on thesubstrate temperature.

As described above, the irradiation of the energy beam (such as anexcimer laser beam) causes alkali metal atoms in the alkali compoundlayer to be partially diffused into the second semiconductor layer 11.Those alkali metal atoms incorporated into the second semiconductorlayer 11 create p-type carriers and thus a contact layer 12 containing ahigh-density p-type impurity is formed on the surface of the secondsemiconductor layer 11. Most of the alkali compound layer on the surfaceis evaporated and lost.

After completion of the above process, the substrate 1 on which thecontact layer has been formed is taken out of the energy beamirradiating apparatus, and is placed into an evaporation apparatus whichis not shown in the figure. A proper metal such as gold is evaporated onthe contact layer 12 and the insulating layer 13 thereby forming thep-side electrode 14 as shown in FIG. 4B (p-side electrode formationstep). Furthermore, for example indium is evaporated on the back surfaceof the substrate 1 thereby forming the n-side electrode 15 (n-sideelectrode formation step). In the above evaporation process, the p-sideelectrode 14 may be formed over the entire surface as shown in FIG. 4Bor only in a particular area corresponding to the contact layer 12(i.e., corresponding to the area where the alkali compound layer hasbeen formed). Herein, “the area corresponding to the contact layer 12”refers to such an area in which the p-side electrode 14 is in contactwith at least a part of the contact layer 12.

After forming the p-side electrode 14 and the n-side electrode 15, thesubstrate 1 is cleaved along planes perpendicular to the longitudinaldirection of the contact layer 12 (perpendicular to the longitudinaldirection of the resonator) at fixed intervals (for example of 600 nm)and reflecting mirror layers are formed on the cleavage surfaces. Thenfurther cleavage is performed so that the substrate is further separatedat the middle between adjacent stripe-shaped contact layers 12 into aplurality of chips (cleavage step). Thus, a semiconductor light emittingdevice in a complete form is obtained as shown in FIG. 1.

In this specific embodiment, the alkali compound layer formation step,the heat treatment step, and the p-side and n-side electrode formationstep are performed in different apparatus. However, these steps may alsobe performed in the same single vacuum apparatus or in separate vacuumapparatus connected to each other via a transfer vacuum path so that theseries of steps from the alkali compound layer formation step to thep-side and n-side electrode formation step is successively performed ina dry ambient containing no water. In this case, because the substrateis maintained in the dry ambient during the above processing steps, nowater is absorbed by the deliquescent alkali compound layer and thus nodegradation occurs in the current-voltage characteristic.

FIG. 7 illustrates an example of a preferable construction of vacuumapparatus. The vacuum apparatus comprises a first evaporation apparatus50 for forming the alkali compound layer 16 a, an energy beamirradiating apparatus 30 similar to the apparatus shown in FIG. 6 forperforming the heating treatment, and a second evaporation apparatus 60for forming the electrodes 14 and 15. The apparatus 50, 30 and 60 areconnected via valves 71, 72, and 73, respectively, to a transfer vacuumchamber 70 having a substrate entrance/exit 74. The evaporationapparatus 50 and 60 have susceptors 51 and 61, respectively, which aredisposed at locations opposite to source materials M in the respectivevacuum chambers 51 and 61 so that various types of materials N can bedeposited on the surface of the substrate 1 placed on the susceptors 52and 62. Using this vacuum apparatus, an alkali compound layer is formedin the evaporation apparatus 50, and the substrate on which the alkalicompound layer has been formed is transferred into the energy beamirradiating apparatus 30 via the transfer vacuum chamber 70. Afterperforming a heat treatment in the energy beam irradiating apparatus 30,the substrate is transferred into the evaporation apparatus 60 via thetransfer vacuum chamber 70 and the electrodes 14 and 15 are formed.

In the vacuum apparatus shown in FIG. 7, the evaporation apparatus 50,the energy beam irradiating apparatus 30, and the evaporation apparatus60 are connected to each other via the transfer vacuum chamber 70.Instead of employing such a vacuum apparatus, evaporation and heattreatment by means of irradiation of an energy beam required in thealkali compound layer formation step, the p-side electrode formationstep, and the n-side electrode formation step may all be performed inthe same single vacuum chamber. Alternatively, evaporation in the alkalicompound layer formation step, the p-side electrode formation step, andthe n-side electrode formation step may be performed in the same singleevaporation apparatus, and the energy beam irradiation may be performedin an energy beam irradiating apparatus separately disposed andconnected to the evaporation apparatus via a transfer vacuum chamber.These methods are advantageous in that a highly-deliquescent alkalicompound deposited on the inner wall of the vacuum chamber is coveredwith metal evaporated in the following step and thus the apparatus canbe controlled into a desirable condition.

After completion of the heat treatment step, there is substantially noresidual alkali compound layer. Therefore, the substrate can be takeninto the atmosphere without encountering any problem. The vacuumapparatus shown in FIG. 7 may be modified such that only the evaporationapparatus 50 and the energy beam irradiating apparatus 30 are connectedto each other via the transfer vacuum chamber 70 or such that a singlevacuum chamber is used only for evaporation of the alkali compound andfor energy beam irradiation. In any case, the alkali compound layerformation step and the heat treatment step-are successively performed ina dry ambient.

The semiconductor light emitting device may also be produced as follows.

FIGS. 8 and 9 illustrate the production processing steps employedherein. At the first step of this production method, the III-V groupbuffer layer 2, the first II-V group buffer layer 3, the second II-Vgroup buffer layer 4, the n-type clad layer 5, the first guide layer 6,the active layer 7, the second guide layer 8, the p-type clad layer 9,the first semiconductor layer 10, and the second semiconductor layer 11are epitaxially grown successively on the substrate 1 as shown in FIG.8A in a similar manner to the multilayer formation step according to theprevious method (multilayer formation step).

Subsequently, the surface of the second semiconductor layer 11 iscleaned in a manner similar to the above-described cleaning stepaccording to the previous method (cleaning step). After that, thesurface of the second semiconductor layer 11 is heated and dried in asimilar manner to the heating and drying step according to the previousmethod (heating and drying step). Then as shown in FIG. 8A, the alkalicompound layer 16 a is formed on the second semiconductor layer 11 in asimilar manner to the alkali compound layer formation step according tothe previous method (alkali compound layer formation step). In thisstep, as in the alkali compound layer formation step according to theprevious production method, the alkali compound layer 16 a may be formedeither over the entire surface of the second semiconductor layer 11 asshown in FIG. 8A, or only in a particular limited areas corresponding tothe stripe-shaped current confining regions which will be formed in thecurrent confining part formation step described later.

After forming the alkali compound layer 16 a, a heat treatment isperformed in a similar manner to the heat treatment step according tothe previous method thereby altering a part, on the side adjacent to thealkali compound layer 16 a, of the second semiconductor layer 11 and atleast a part, in contact with the second semiconductor layer 11, of thealkali compound layer 16 a thus forming the contact layer 12, as shownin FIG. 8B (heat treatment step).

After completion of the heat treatment step, as in the current confiningpart formation step according to the previous method, the contact layer12 and the second semiconductor layer 11 are selectively removed using aresist film M formed on the contact layer 12 as a mask as shown in FIG.9A thereby converting the contact layer 12 and the second semiconductorlayer 11 into the form of a plurality of parallel stripes so that theyact as current confining parts. In FIG. 9A, only one of these stripes ofthe contact layer 12 and the second semiconductor layer 11 is shown in arepresentative fashion (FIG. 9B is also represented in a similarfashion). After that, as in the current confining part formation stepaccording to the previous method, an insulating material such asaluminum oxide is evaporated over the entire surface, and the resistfilm M is then removed (lifted off) together with the part of theinsulating material evaporated on the resist film M thereby forming theinsulating layer 13 as shown in FIG. 9B (current confining partformation step).

After the formation of the current confining parts, as in the p-sideelectrode formation step and as in the n-side electrode formation stepaccording to the previous method, the p-side electrode 14 is formed onthe contact layer 12 and the insulating layer 13 (p-side electrodeformation step) and the n-side electrode 15 is formed on the backsurface of the substrate 1 (n-side electrode formation step). Afterthat, the substrate 1 is cleaved (cleavage step) in a similar manner tothe cleavage step according to the previous method. Thus, thesemiconductor light emitting device in the complete form shown in FIG. 1is obtained.

Also in this production method, the alkali compound layer formation stepand the heat treatment step may be performed in the same singleapparatus or in different vacuum apparatus connected to each other via atransfer vacuum chamber so that the process from the alkali compoundlayer formation step to the heat treatment step is successivelyperformed in a dry ambient containing no water.

The operation of the semiconductor light emitting device produced in theabove process is described below.

When a voltage is applied between the n-side electrode 15 and the p-sideelectrode 14 of the semiconductor light emitting device, a current isinjected into the active layer 7. In the active layer 7, electron-holerecombination occurs whereby light is emitted. Since the semiconductorlight emitting device has the low-resistance contact layer 12 formedbetween the p-side electrode 14 and the second semiconductor layer 11,the electric resistance at the interface between the p-side electrode 14and the contact layer 12 becomes low and thus the voltage drop acrossthe interface also becomes low. As a result, the semiconductor lightemitting device can be operated with low electric power withoutgenerating a significantly great amount of heat, and thus a long devicelife can be achieved.

To verify the advantageous effects of the semiconductor light emittingdevice according to the present invention, a device having a differentstructure was also fabricated and comparison was made. The device forcomparison was fabricated as follows. A II-VI group compoundsemiconductor (ZnSe) layer was grown on a substrate, and a thin layer(with a thickness of about 200 Å) of an alkali compound (Na₂Se) wasevaporated thereon. After performing a heating process using the energybeam irradiating apparatus shown in FIG. 6, electric resistance wasmeasured. In the above heating process, an excimer laser was employed asthe energy beam generator 41, and a pulse train of excimer laser beamwas applied to the device from the side on which the alkali compound isformed. The wavelength was 248 originating from KrF, the pulse width wasset to 20 ns, the output power was 20 to 120 mJ/cm², and the number ofpulses was varied within a certain range. The inside of the reactionchamber 31 was filled with a nitrogen ambient at a pressure of 2 atm,and the substrate was maintained at room temperature.

The result is shown in FIG. 10 which represents the dependence of theelectric resistance on the number of pulses applied. As can be seen fromFIG. 4, the electric resistance decreases with the number of appliedpulses. This is because alkali metal atoms (sodium atoms in this case)in the alkali compound are diffused into the II-VI group compoundsemiconductor (ZnSe) and they behave as a p-type impurity therein.

To verify the effects of the heating and drying process according to themethod of producing a semiconductor light emitting device of the presentembodiment, a semiconductor light emitting device having the structureshown in FIG. 1 was produced according to the method of the presentembodiment, and the current-voltage characteristic thereof wasevaluated. Furthermore, another sample was prepared by performing theprocess in the same manner as in the present embodiment from thebeginning to the heat treatment step, then forming a p-side electrode 14in a particular limited area corresponding to the contact layer 12, andfinally cleaving the substrate along planes perpendicular to thelongitudinal direction of the contact layer 12 at intervals of 1 mm. Thecurrent-voltage characteristic between the p-side electrodes of adjacenttwo semiconductor light emitting devices is evaluated. Furthermore, forthe purpose of comparison, still another sample was prepared byperforming the process which is the same as that employed in the presentembodiment except that the heating and drying process is omitted, andthe current-voltage characteristic of the obtained semiconductor lightemitting device and the current-voltage characteristic between thep-side electrodes of adjacent semiconductor light emitting devices wereevaluated.

In these samples, the width of the contact layer 12 (i.e., the width ofthe second semiconductor layer 11) was set to 10 μm, and Na₂Se wasevaporated in the alkali compound layer formation step. In the heattreatment step, a KrF excimer laser beam with a wavelength of 248 nm wasemployed wherein the pulse width, the output power, and the number ofpulses were set to 20 ns, 90 mJ/cm², and 5, respectively. The heattreatment step was performed in the reaction chamber 31 filled with annitrogen ambient at a pressure of 2 atm while the substrate 1 ismaintained at room temperature.

The results are shown in FIGS. 11 to 14. FIG. 11 illustrates thecurrent-voltage characteristic of the semiconductor light emittingdevice produced according to the method of the present embodiment. FIG.12 illustrates the current-voltage characteristic of the semiconductorlight emitting device prepared for the purpose of comparison. FIG. 13illustrates the current-voltage characteristic between the p-sideelectrodes of adjacent light emitting devices produced according to thepresent embodiment. FIG. 14 illustrates the current-voltagecharacteristic between the p-side electrodes of adjacent light emittingdevices prepared for the purpose of comparison.

As can be seen from FIG. 11, the semiconductor light emitting devicesubjected to the heating and drying process has a good current-voltagecharacteristic in which a current of 50 mA required for operating thesemiconductor light emitting device is obtained by applying a lowvoltage of about 6 V. In contrast, as can be seen from FIG. 12, thesemiconductor light emitting device subjected to no heating and dryingprocess has a poor current-voltage characteristic in which a largeenough current cannot be obtained when a voltage is applied.Furthermore, as can be seen from FIG. 13 in the semiconductor lightemitting device subjected to the heating and drying process, a goodohmic contact is obtained at the interface between the p-side electrode14 and the contact layer 12. In contrast, as can be seen from FIG. 14 inthe semiconductor light emitting device subjected to no heating anddrying process, an ohmic contact is not obtained. Thus it can beunderstood that the electric resistance at the interface between thep-side electrode 14 and the contact layer becomes low and thus thevoltage drop across the interface also becomes low as a result of theheating and drying process.

Furthermore, in order to investigate the influence of energy beamirradiation conditions in the heat treatment process on the change inthe characteristics, semiconductor light emitting devices were producedunder various irradiation conditions (excimer laser beam irradiationconditions) while employing the same conditions as those employed toproduce the semiconductor light emitting devices shown in FIGS. 11 and13, and the current-voltage characteristic and also the current-voltagecharacteristic between adjacent p-side electrodes were evaluated.

In the above production, the excimer laser beam irradiation wasperformed in such a manner that ten pulses with output power of 29mJ/cm² were applied at the first stage, and then one pulse with outputpower of 100 mJ/cm² was further applied at the second stage, in whichthe pulse width was fixed to 20 ns.

Results are shown in FIGS. 15 and 16. FIG. 15 illustrates thecurrent-voltage characteristic of the semiconductor light emittingdevice subjected to the two-stage laser beam irradiation process. FIG.16 illustrates the current-voltage characteristic between adjacentp-side electrodes of the semiconductor light emitting device subjectedto the two-stage laser beam irradiation process. As can be seen fromFIG. 15, if the excimer laser beam irradiation is performed in such amanner that the device is first irradiated with a low-energy beam at thefirst stage and then irradiated with a high-energy beam at the secondstage, the resultant device exhibits a good characteristic in which acurrent of 50 mA can be obtained by applying a voltage as low as about5.3 V. Compared with the device subjected to the irradiation of aplurality of pulses all having equal energy (a voltage of about 6 V isrequired to obtain a current of 50 mA; refer to FIG. 11), this devicecan be operated at a still lower voltage. Furthermore, as can be seenfrom FIG. 16, the device subjected to the two-stage excimer laser beamirradiation has a good ohmic contact at the interface between the p-sideelectrode 14 and the contact layer 12.

In the semiconductor light emitting device according to the presentembodiment of the invention, as described above, the electric resistanceat the interface between the p-side electrode 15 and the contact layer12 and thus the voltage drop across the interface are reduced as aresult of the formation of the contact layer, containing at least aproduct of thermal reaction of the second semiconductor layer 11 with analkali compound, between the second semiconductor layer 11 and thep-side electrode 15. As a result, the semiconductor light emittingdevice can be operated with low power and with a reduced amount of heatgeneration thereby achieving an increased device life.

In other words, these advantages of the semiconductor light emittingdevice are achieved by forming the contact layer 12 between the secondsemiconductor layer 11 and the p-side electrode 14 in such a manner thatat least a part of the contact layer 12 consists of a II-VI groupcompound semiconductor containing an alkali metal element as a p-typeimpurity.

In this semiconductor light emitting device in which the electricresistance between the p-side electrode and the II-VI group compoundsemiconductor layer is reduced by forming the contact layer 12, thestructure is simplified compared with the structure of conventionaldevices having a superlattice layer or a composition-graded layer. Thismakes it possible to produce a semiconductor light emitting devicehaving good crystal quality, and productivity particularly in massproduction can be improved.

In the method of producing a semiconductor light emitting deviceaccording to the present embodiment of the invention, the alkalicompound layer is formed on the second semiconductor layer 11 and then aheat treatment is performed thereby easily forming the contact layer 12according to the embodiment thus realizing the semiconductor lightemitting device according to the present embodiment of the invention.

Furthermore, in this method of producing a semiconductor light emittingdevice, before the process of forming the alkali compound layer, theheating and drying process is performed so as to remove water from thesurface thereby ensuring that the contact layer 12 has a goodcurrent-voltage characteristic.

Second Embodiment

FIG. 17 illustrates a second embodiment of a semiconductor lightemitting device according to the invention. In this second embodiment,there is shown another specific example of II-VI group compoundsemiconductors used to form the second II-VI group buffer layer 4 of aII-VI compound semiconductor, the n-type clad layer 5, the first guidelayer 6, the second guide layer 8, the p-type clad layer 9, and thefirst semiconductor layer 10 which have different compositions fromthose employed in the first embodiment. Herein, similar elements tothose in the first embodiment are denoted by similar reference numerals,and they are not described in further detail.

The second II-VI group buffer layer 4 is formed for example of an n-typeZnBeSe mixed crystal doped with chlorine serving as an n-type impurity.The clad layer 5 is formed for example of an n-type ZnMgBeSe mixedcrystal doped with chlorine serving as an n-type impurity. The firstguide layer 6 is formed for example of a ZnBeSe mixed crystal doped withchlorine serving as an n-type impurity or doped with no impurities. Thefirst guide layer 6 is formed for example of a ZnBeSe mixed crystaldoped with chlorine serving as an n-type impurity or doped with noimpurities. The p-type clad layer 9 is formed for example of a p-typeZnMgBeSe mixed crystal doped with nitrogen serving as a p-type impurity.The first semiconductor layer 10 is formed for example of a p-typeZnBeSe mixed crystal doped with nitrogen serving as a p-type impurity.

The semiconductor light emitting device having the above structure maybe produced in a similar manner to that according to the firstembodiment, and operates in a similar manner to that according to thefirst embodiment. This means that similar advantageous effects obtainedin the semiconductor light emitting device according to the firstembodiment can also be achieved in various structures having modifiedcompositions of the respective II-VI group compound semiconductorlayers, as long as the p-side electrode 15 is formed on a similarcontact layer 12.

Third Embodiment

FIG. 18 illustrates a third embodiment of a semiconductor light emittingdevice according to the present invention. This semiconductor lightemitting device has a similar structure to that of the first embodimentexcept that the second semiconductor layer 11 is removed and the contactlayer 12 is formed directly on the first semiconductor layer 10. Thus,similar elements to those in the first embodiment are denoted by similarreference numerals, and they are not described in further detail.

The semiconductor light emitting device having the above structure maybe produced in a similar manner to that according to the firstembodiment, and operates in a similar manner to that according to thefirst embodiment. That is, similar advantages to those obtained in thesemiconductor light emitting device according to the first embodimentare also achieved in this third embodiment by means of forming thep-side electrode 14 on the contact layer 12 containing the alkali metalelement serving as the p-type impurity.

The semiconductor light emitting device according to the secondembodiment may also be constructed into a form in which there is nosecond semiconductor layer 11 as is the case in the third embodiment.

Fourth Embodiment

FIG. 19 illustrates a fourth embodiment of a semiconductor lightemitting device according to the present invention. This semiconductorlight emitting device has a similar structure to that of the firstembodiment except that a third semiconductor layer 17 is additionallyformed between the second semiconductor layer 11 and the contact layer12. Thus, similar elements to those in the first embodiment are denotedby similar reference numerals, and they are not described in furtherdetail.

The third semiconductor layer 17 is made up of a II-VI group compoundsemiconductor superlattice layer or a graded layer of a II-VI groupcompound semiconductor in which the composition is changed in thedirection of layer growth. For example, the third semiconductor layer 17is made up of a superlattice layer comprising ZnSe layers serving asfirst II-VI group semiconductor layers and ZnTe layers serving as secondII-VI group semiconductor layers wherein both ZnSe layers are doped witha p-type impurity such as nitrogen and they are disposed alternately.Alternatively, the third semiconductor layer 17 may be made up of agraded layer consisting of zinc as the II-group element and selenium andtellurium as the VI-group elements wherein the graded layer is dopedwith a p-type impurity such as nitrogen and wherein theselenium-tellurium composition ratio is varied in the direction of layergrowth.

Specific preferable examples of the superlattice layer include a(ZnSe)m(ZnTe)n superlattice layer (m>5, n=1, 2) (refer to JapanesePatent Laid-Open No. 6-5920) and a superlattice layer in which one ortwo atomic layers of each ZnSe layer consisting of 10 atomic layers arereplaced by an atomic layer of ZnTe wherein only the ZnTe layers areδ-doped with nitrogen serving as a p-type impurity at a density of about1×10²⁰ cm⁻³ so that a carrier concentration of about 1×10¹⁹ cm⁻³ isobtained over the entire superlattice layer.

As well as the second semiconductor layer 11 and the contact layer 12,the third semiconductor layer 17 is also formed into a stripe shape witha width of for example 10 μm which serves as a current confining part inwhich a current is confined.

The semiconductor light emitting device having the above structure canbe produced in a similar manner to that of the first embodiment. In thissemiconductor light emitting device, the third semiconductor layer 17formed between the contact layer 12 and the second semiconductor layer11 brings about a further reduction in the electric resistance comparedto the electric resistance of the semiconductor light emitting deviceaccording to the first embodiment.

In the semiconductor light emitting device according to the presentembodiment of the invention, as described above, a further reduction inthe electric resistance and thus a further reduction in the voltage dropare obtained as a result of the formation of the third semiconductorlayer 17 in the form of a superlattice layer or a graded layer betweenthe contact layer 12 and the second semiconductor layer 11 (i.e.,between the contact layer 12 and the p-type clad layer 9). As a result,the semiconductor light emitting device can be operated with furtherlower power, and a further reduction in the amount of heat generation isachieved. Thus a further increase in the device life is achieved.

The semiconductor light emitting device according to the secondembodiment may also be constructed into a form in which there isprovided a third semiconductor layer 17 between the second semiconductorlayer 11 and the contact layer 12, as is the case in this fourthembodiment.

Fifth Embodiment

FIG. 20 illustrates a fifth embodiment of a semiconductor light emittingdevice according to the invention. This semiconductor light emittingdevice has a similar structure to that of the first embodiment exceptthat there is an alkali compound layer 16 b between the insulating layer13 and the p-side electrode 14 and that the p-side electrode 14 has adiffusion barrier layer 14 a located on the side adjacent to the contactlayer 12. Thus, similar elements to those in the first embodiment aredenoted by similar reference numerals, and they are not described infurther detail.

The alkali compound layer 16 b is formed so that it is used in a laterprocessing step to form the contact layer 12, as will be describedlater. The alkali compound layer 16 b may be a compound of an alkalimetal and a VI-group element, or alkali metal and phosphorus, orotherwise alkali metal, a VI-group element and phosphorus. Morespecifically, the alkali compound layer 16 b may be a similar compoundto that employed in the first embodiment. In this embodiment, althoughthe alkali compound layer 16 b is formed between the insulating layer 13and the p-side electrode 14, the alkali compound layer can be vaporizedin the following steps as will be described later, and thus there can beno alkali compound layer.

The diffusion barrier layer 14 a of the p-side electrode 14 preventselements constituting the p-side electrode 14 from being diffused towardthe contact layer 12 during the subsequent processing steps describedlater, and also prevents the alkali compound layer 16 b from beingvaporized during the heat treatment. Preferably, the diffusion barrierlayer 14 a is made of a high-melting point metal or a metal containing ahigh-melting point metal. Herein, the high-melting point metal refers toa metal having a melting point of 1000° C. or higher, such as gold (Au),copper (Cu), manganese (Mn), molybdenum (Mo), niobium (Nb), nickel (Ni),palladium (Pd), platinum (Pt), tantalum (Ta) and titanium (Ti). That is,in a preferable mode, the diffusion barrier layer 15 a consists of atleast one of these metals. More preferably, a metal having a meltingpoint higher than 1200° C. is employed, and titanium is most preferable.

The semiconductor light emitting device having the above structure canbe produced in a similar manner to that of the first embodiment exceptthat a heat treatment is performed after forming the p-side electrode14. In the p-side electrode formation step, for example a metal (such astitanium) used to form the diffusion barrier layer 14 a is firstevaporated to a thickness of for example 10 nm, and then for examplegold is evaporated to a thickness of 300 nm. The diffusion barrier layerprevents the alkali compound from being vaporized in the laterprocessing steps and also prevents elements constituting the p-sideelectrode 14 from being diffused toward the contact layer 12.

There is a possibility that the p-side electrode 14 is partially lostduring the heat treatment process. To deal with such a loss of thep-side electrode 14, the p-side electrode 14 may be formed again afterthe heat treatment process (p-side electrode re-forming step). In thiscase, it is not required to form the diffusion barrier layer 14 a.

The semiconductor light emitting device having the above structure hassimilar advantages to those achieved in the first embodiment. That is,advantages similar to those achieved in the first embodiment are alsoachieved in this fifth embodiment as a result of the formation of thep-side electrode 14 on the contact layer 12 containing the alkali metalelement serving as the p-type impurity.

To verify the advantageous effects of the semiconductor light emittingdevice according to the present invention, a device having a differentstructure was also fabricated and comparison was made. The device forcomparison was fabricated as follows. A II-VI group compoundsemiconductor (ZnSe) layer was grown on a substrate, and a thin layer(with a thickness of about 200 Å) of an alkali compound (Na₂Se) wasevaporated thereon. Furthermore, metals (titanium with a thickness of 10nm and gold with a thickness of 300 nm) were evaporated thereon therebyforming an electrode. After performing a heating process using theenergy beam irradiating apparatus shown in FIG. 6, electric resistancewas measured. The heat treatment process was performed under the sameconditions as in the experiment performed in the first embodiment. As inthe first embodiment, the electric resistance decreases with the numberof pulses.

The semiconductor light emitting device according to any of secondthrough fourth embodiments may also be constructed into a similar formusing a similar production process.

Sixth Embodiment

In this sixth embodiment of the invention, an optical device providedwith a semiconductor light emitting device according to the presentinvention is disclosed. As an example of such an optical device, anoptical disk reproducing apparatus is described below.

FIG. 21 illustrates the construction of an optical disk reproducingapparatus 100. this optical disk reproducing apparatus 100 is designedto reproduce information recorded on an optical disk 200. For thispurpose, the optical disk reproducing apparatus 100 has a semiconductorlight emitting device 101 for emitting light with a wavelength betweengreen and blue. A semiconductor light emitting device according to anyof first through fifth embodiments described above may be employed asthe semiconductor light emitting device 101. The optical diskreproducing apparatus 100 further includes a known optical system fortransmitting a light beam output from the semiconductor light emittingdevice 101 to the optical disk 200. The optical system comprises acollimator lens 102, a beam splitter 103, a λ/4 plate 104, an objectivelens 105, a detection lens 106, a signal light detection photosensor107, and a signal light reproducing circuit 108.

In this optical disk reproducing apparatus 100, the light emitted fromthe semiconductor light emitting device 101 is collimated by thecollimator lens 102, and is incident on the λ/4 plate 104 via the beamsplitter 103. The λ/4 plate 104 adjusts the polarization of the light,and the resultant light is focused onto the optical disk 200 via theobjective lens 105. Signal light 200 is produced as a result ofreflection by the optical disk 200, and is incident on the beam splitter103 via the objective lent 105 and the λ/4 plate 104. The beam splitter103 reflects the signal light 200 toward the detection lens 106. Afterpassing through the detection lens, the signal light 200 is incident onthe signal light detection photosensor 107. The signal light detectionphotosensor 107 converts the received signal light to a correspondingelectric signal, which is transferred to the signal light reproducingcircuit 108. Thus, information written on the optical disk 200 isreproduced from the signal light reproducing circuit 108.

Since the semiconductor light emitting device 101 used herein has thestructure according to the invention, the electric resistance at theinterface between the p-side electrode 14 and the contact layer 12 isreduced, and thus it can be operated with reduced electric power and anincreased device life is achieved. As a result of the application of thesemiconductor light emitting device 101 having such advantages to theoptical disk reproducing apparatus 100, the operating power of theoptical disk reproducing apparatus 100 is reduced, and the life of theoptical disk 200 and that of the optical disk reproducing apparatus 100are improved.

Although in this specific embodiment the semiconductor light emittingdevice according to the invention is applied to the optical diskreproducing apparatus, it may also be applied to other various opticalapparatus such as an optical disk recording apparatus, an optical diskrecording/reproducing apparatus, an optical communication apparatus, adisplay device, an optical analysis or detection instrument, and adevice provided with a semiconductor laser which is installed on a carand is operated in a high-temperature environment.

Although the present invention has been described above with referenceto specific embodiments, the invention is not limited to the details ofthese embodiments, but various modifications and changes are possible.That is, in the respective embodiments described above, the first II-VIgroup compound semiconductor 3, the second II-VI group buffer layer 4,the n-type clad layer 5, the first guide layer 6, the active layer 7,the second guide layer 8, the p-type clad layer 9, the firstsemiconductor layer 10, and the second semiconductor layer 11 are madeof specific kinds of II-VI group compound semiconductors. However, inthe present invention, the respective layers may also be formed of otherproper kinds of II-VI group compound semiconductors (comprising at leastone II-group element selected from the group consisting of zinc,magnesium, cadmium, manganese, mercury, and beryllium and at least oneVI-group element selected from the group consisting of oxygen; selenium,sulfur, and tellurium).

Furthermore although in the embodiments described above, the activelayer 7 is sandwiched between the guide layers 6 and 8 and they arefurther sandwiched between the guide layers 6 and 8, the active layer 7may be disposed for example directly between clad layers withoutinterposing guide layers.

Furthermore, although in the above embodiments, the current confiningpart are constructed with the stripe-shaped contact layer 12, thecurrent confining part constructed in a different fashion also fallswithin the scope of the present invention.

Still furthermore, although in the above embodiments, the contact layer12 is formed by means of irradiation of an energy beam, the presentinvention also includes, in its scope, other methods of forming thecontact layer 12 using different heating processes.

Still furthermore, although in the above embodiments, the contact layer12 contains a product of thermal reaction between an alkali compound anda II-VI group compound semiconductor or contains an alkali compound aswell as such a thermal reaction product, similar advantageous effectscan also be achieved by forming the contact layer 12 so that at least apart of it is formed of a II-VI compound semiconductor containing analkali metal element serving as a p-type impurity. That is,semiconductor light emitting devices having a contact layer 12 formed bymeans of a process other than heat treatments also fall within the scopeof the invention.

Still furthermore, although in the above embodiments the respectivelayers are grown on the substrate 1 using the solid source MBEtechnique, gas source MBE or MOCVD (metal organic chemical vapordeposition) or other techniques may also be employed to grow the layers.

In the semiconductor light emitting device according to an aspect of thepresent invention, as described above, the contact layer is disposedbetween the p-type clad layer and the p-side electrode so that at leasta part of the contact layer is formed of a II-VI compound semiconductorcontaining an alkali metal element serving as a p-type impurity therebyachieving a reduced electric resistance at the interface between thep-side electrode and the contact layer and thus a reduced voltage dropacross the interface. Therefore, the semiconductor light emitting devicecan be operated with low electric power, and thus the amount of heatgenerated is reduced. Thus an improved device life is achieved.Furthermore, the device structure is simplified and it becomes possibleto easily produce a semiconductor light emitting device having goodcrystal quality. As a result, productivity particularly in massproduction is improved.

In the semiconductor light emitting device according to another aspectof the present invention, as described above, the contact layer isdisposed between the p-type clad layer and the p-side electrode whereinthe contact layer contains at least a product of thermal reactionbetween an alkali compound and a II-VI group compound semiconductor sothat the electric resistance at the interface between the p-sideelectrode and the contact layer is reduced to a similar level to thatachieved in the semiconductor light emitting device according to theprevious aspect of the invention. Thus, similar advantageous effects arealso achieved.

In the method of producing a semiconductor light emitting deviceaccording to an aspect of the invention, after forming the alkalicompound layer, the p-side electrode is formed in the area correspondingto the alkali compound layer. This makes it possible to easily produce asemiconductor light emitting device according to the invention. That is,semiconductor light emitting devices according to the invention can berealized by this production method.

Furthermore, since the optical device according to the invention has asemiconductor light emitting device according to the invention, theelectric power required to operate the optical device is reduced and animproved life is achieved as a result of the improvement of powerconsumption and life of the semiconductor light emitting deviceemployed.

What is claimed is:
 1. A semiconductor light emitting device comprisingat least an n-type clad layer, an active layer, and a p-type clad layerformed into a multilayer structure using a II-VI compound semiconductorcontaining at least one II-group element selected from the groupconsisting of zinc (Zn), magnesium (Mg), beryllium (Be), cadmium (Cd),manganese (Mn), and mercury (Hg) and at least one VI-group elementselected from the group consisting of oxygen (O), sulfur (S), selenium(Se), and tellurium (Te), said semiconductor light emitting device alsocomprising a p-side electrode electrically connected to said p-type cladlayer, said semiconductor light emitting device being characterized inthat: a contact layer is provided between said p-type clad layer andsaid p-side electrode, said contact layer being formed, at least inpart, of a II-VI group semiconductor containing an alkali metal elementserving as a p-type impurity; and said p-side electrode includes, atleast in a part thereof, a diffusion barrier layer for preventing anelement constituting said p-side electrode from being diffused towardsaid contact layer.
 2. A semiconductor light emitting device accordingto claim 1, wherein said diffusion barrier layer contains at least oneelement selected from the group consisting of gold (Au), copper (Cu),manganese (Mn), molybdenum (Mo), niobium (Nb), nickel (Ni), palladium(Pd), platinum (Pt), tantalum (Ta) and titanium (Ti).
 3. A semiconductorlight emitting device comprising at least n-type clad layer, an activelayer, and a p-type clad layer formed into a multilayer structure usinga II-VI compound semiconductor containing at least one II-group elementselected from the group consisting of zinc (Zn), magnesium (Mg),beryllium (Be), cadmium (Cd), manganese (Mn), and mercury (Hg) and atleast one VI-group element selected from the group consisting of oxygen(O), sulfur (S), selenium (Se), and tellurium (Te), said semiconductorlight emitting device also comprising a p-side electrode electricallyconnected to said p-type clad layer, said semiconductor light emittingdevice being characterized in that: a contact layer is provided betweensaid p-type clad layer and said p-side electrode, said contact layerbeing formed at least in part of a II-VI group semiconductor layercontaining an alkali metal element serving as p-type impurity; and saidsemiconductor layer and the contact layer are disposed between laterallyopposed insulating layers, and within a thickness dimension of saidlayers, wherein said p-side electrode includes, at least in a partthereof, a diffusion barrier layer for preventing an elementconstituting said p-side electrode from being diffused toward saidcontact layer.
 4. A semiconductor light emitting device according toclaim 3, wherein said diffusion barrier layer contains at least oneelement selected from the group consisting of gold (Au), copper (Cu),manganese (Mn), molybdenum (Mo), niobium (Nb), nickel (Ni), palladium(Pd), platinum (Pt), tantalum (Ta) and titanium (Ti).
 5. An opticaldevice including a semiconductor light emitting device, saidsemiconductor light emitting device comprising at least an n-type cladlayer, an active layer, and a p-type clad layer formed into a multilayerstructure using a II-VI compound semiconductor containing at least oneII-group element selected from the group consisting of zinc (Zn),magnesium (Mg), beryllium (Be), cadmium (Cd), manganese (Mn), andmercury (Hg) and at least one VI-group element selected from the groupconsisting of oxygen (O), sulfur (S), selenium (Se), and tellurium (Te),said semiconductor light emitting device also comprising a p-sideelectrode electrically connected to said p-type clad layer, wherein, acontact layer is provided between said p-type clad layer and said p-sideelectrode, said contact layer being formed, at least in part, of a II-VIgroup semiconductor containing an alkali metal element serving as ap-type impurity; and said p-type electrode includes, at least in partthereof, a diffusion barrier layer for preventing an elementconstituting said p-side electrode from being diffused toward saidcontact layer.